1. Field of the Invention
The present invention relates to an apparatus and a method for preamble Pseudo Noise (PN) code allocation in a broadband wireless communication system, and more particularly to an apparatus and a method for allocating different preamble PN codes to respective Frequency Allocations (FAs) in a base station of a broadband wireless communication system which amplifies a plurality of FAs with one amplifier.
2. Description of the Related Art
The IEEE 802.16e system basically adopts a cellular method and supports a frequency reuse factor of 1. For this reason, it is possible to share an identical frequency with adjacent cells. Therefore, a terminal within the system must be able to identify a base station to which it belongs from adjacent base stations which use an identical frequency. For identification, each base station inserts a preamble Pseudo Noise (PN) code, i.e., its inherent PN code into a preamble of each frame to be transmitted to a terminal.
The number of the preamble PN codes defined in the IEEE 802.16e system reference is 114. Respective preamble PN codes have preamble PN indexes from 0 to 113, and each preamble PN index has an inherent preamble PN code, an ID cell and a segment number as shown in FIG. 1. Each base station is allocated with one of the preamble PN codes and broadcasts it to a terminal. Then the terminal interprets the received preamble PN code and thereby checks an IDcell and a segment number of corresponding base station. Here, the IDcell has 32 different values from 0 to 31, and the segment number has three different values from 0 to 2. Therefore, all preamble PN codes cannot have their inherent combinations of an IDcell and a segment number. Among the 114 preamble PN codes, code 0 through code 95 have their inherent combinations of an IDcell and a segment number, whereas code 96 through code 113 have the same combinations as those of code 0 through code 95.
The preamble PN code is used when a terminal turns on and initially tries to connect to a system, in order to search a base station to perform a communication. The terminal matches a received signal to the 114 preamble PN codes, synchronizes an initial time to the most well-matched preamble PN code, and starts a communication with a base station having the corresponding preamble PN index. The preamble PN code is also used to search an adjacent base station when the terminal needs a handover.
In addition, the preamble PN code transmits an IDcell and a segment number of corresponding base station through a preamble PN index. The IDcell and segment number are important parameters used in a sub-carrier randomization, a sub-channel permutation, a cluster renumbering, a sub-channel renumbering, and so on. A terminal checks a preamble PN index of a base station and thereby obtains an IDcell and a segment number of the corresponding base station.
When an identical preamble PN index is allocated to adjacent base stations, the terminal cannot perform an initial search for a base station. Hence, the preamble PN index must be allocated to each base station in such a way that minimizes a replicated allocation to adjacent base stations. In addition, since the IDcell and the segment number are also important parameters for identifying a base station, they also should be allocated to each base station in such a way that minimizes the replicated allocation to adjacent base stations.
Meanwhile, a Radio Frequency (RF) power amplifier in a broadband wireless communication system is a significant component which takes considerable parts in material cost, space, and power consumption. The power amplifier is classified into a Single Carrier Power Amplifier (SCPA) and a Multi-Carrier Power Amplifier (MCPA) according to a bandwidth of an input signal. In general, the MCPA is more expensive than the SCPA in order to maintain linearity. However, when a system uses a plurality of FAs, using one MCPA may be more economical than using a plurality of SCPAs.
Many methods for allocating a preamble PN index to each base station in a broadband wireless communication system have been proposed. However, those methods are applicable to a system using one FA, and when the methods are directly applied to a system using a plurality of FAs, an identical preamble PN index is allocated to all FAs in a base station. When the base station amplifies each FA by a separate power amplifier, that is, when a separate SCPA is used for each FA, it doesn't cause a problem even though an identical preamble PN index is allocated to the FAs. However, when a plurality of FAs are amplified by one power amplifier, that is, when the MCPA is used, allocation of identical preamble PN index to the FAs may cause a problem.
When an identical preamble PN index is used for a plurality of FAs, an identical preamble PN code is transmitted from preamble symbols to the FAs. That is, all FAs transmit an identical signal. In this case, a Peak-to-Average Power Ratio (PAPR) of a base station is significantly increased as compared to the case when each FA transmits a different signal. In a downlink zone except in a preamble, different signals are transmitted every frame. Therefore, it has little possibility of transmitting an identical signal by different FAs. However, since the preamble transmits always an identical signal, when the preamble PN index values used in respective FAs are identical, the PAPR may be greatly increased. The rise of the PAPR increases an output span in which a power amplifier should maintain linearity, thereby increasing the price of the power amplifier. That is, when a base station operating a plurality of FAs using the MCPA uses an identical preamble PN index for the FAs, the material cost of the power amplifier may be increased due to a rise of the PAPR. Accordingly, it is better to use different preamble PN index values in one base station.
If a system allocates preamble PN indexes to each base station in consideration of a preamble PN index replication for the FAs within a base station as well as of the preamble PN index replication in adjacent base stations, the above-mentioned problem may be solved. One easy way is to divide a preamble PN set by each FA. For example, when a system uses three FAs, the preamble PN index is divided into three groups, and each FA uses only the preamble PN index belonging to one group. However, the number of the preamble PN indexes is 114, and the ID cell and the segment number which are the components of the preamble PN index are just 32 and 3, respectively. Therefore, even when only one FA is used, it is difficult to allocate the preamble PN index without replication among adjacent base stations. Furthermore, if the FA is considered, it becomes more difficult to allocate the preamble PN index, and it may increase the possibility of parameter replication among adjacent base stations.
For this reason, a preamble PN index allocation method in a broadband wireless communication system using a plurality of FAs should satisfy the following two requirements simultaneously. First, a different preamble PN index must be allocated to each FA within a base station. Second, the allocation to respective FAs should not deteriorate an allocation performance of the preamble PN index, IDcell and segment, as compared to the FA allocation in a system using one FA. Accordingly, in order to reduce the price of a power amplifier in a broadband wireless communication system using a plurality of FAs, there is needed a preamble PN index allocation method which does not deteriorate the performance of the preamble PN index allocation to each FA while using different preamble PN indexes for respective FAs in a base station.